Despite confinement, crop genes can spread fast to wild
With the slim chance that farmers will stop planting crops containing genes from other organisms, researchers have started to develop strategies that trap these foreign genes, reducing the risk that theyll spread to wild relatives.
But an investigation by scientists from the University of Wisconsin-Madison and the University of Minnesota-St. Paul shows that these containment strategies can quickly fail.
Using mathematical models, the team of scientists explored the effectiveness of proposed containment strategies to inhibit the escape of transgenes - genetic information from other organisms thats artificially inserted into crop plants to make them more resistant to pests, herbicides or climate conditions. The findings, published in the March issue of Ecology Letters, show a high probability that leakage can occur much sooner than expected.
"Lots of people are worried about gene flow from cultivated crops to wild relatives," says Ralph Haygood, a UW-Madison postdoctoral fellow and the lead author of paper. Transgene escape - when artificially inserted genes flow from crops to nearby wild populations and become a permanent feature of their genomes - is worrisome, he says, because it can change the genetic make-up of wild populations, sometimes eliminating genes that could be used to improve crops, and possibly turning these wild populations into aggressive weeds.
The goal, then, is to develop strategies to prevent transgene escape.
"Environmentalists say we should stop planting transgenic crops, but thats not going to happen," says the Wisconsin researcher. "Aside from not growing transgenic crops near sexually-compatible wild relatives, we need to investigate ways to reduce the risk."
Strategies currently being developed involve gene containment, where the artificially inserted genes are confined and, theoretically, inhibited from escaping or being favored in wild populations. For example, the technique called the "exorcist" induces certain chemical reactions inside the plant cell that pulls out and eliminates the transgene once the plant no longer needs it. Another technique involves inserting the artificial gene near a gene thats bad for the plant under wild conditions, making it unlikely that the transgene, should it escape, will spread in the wild population.
The gene-confinement strategy closest to commercialization, says Haygood, involves inserting genetic information into the DNA of the chloroplast, a part of the plant cell that contains its own genome. An advantage of this strategy is that chloroplast DNA - and any artificial genetic information it includes - is rarely transmitted through the plants pollen, the main vehicle for transporting genetic information to nearby wild relatives.
"This technique is being greeted as a panacea that could make the whole problem of transgene escape go away," says Haygood. But, as he points out, "it has been shown that chloroplast DNA transmission through pollen can occur at a low rate." He asks, "How much does that matter?"
Given that this gene containment strategy is not failsafe - suggesting that transgene escape is inevitable, given enough time - the researchers investigated the rate at which artificially inserted genes, confined by some of the strategies mentioned above, could reach and become fixed in wild populations.
"For each strategy, there is the possibility of transgene leakage," explains Haygood. "The question shouldnt be whether or not transgene escape will happen. It should be how long will it take."
To answer this question, Haygood, Anthony Ives from UW-Madison and David Andow from the University of Minnesota-St. Paul developed a mathematical model based on factors controlling gene flow from crop plants to wild relatives. The factors include the rate of transgene leakage, the rate of pollen flow, the size of the wild population and the effects of the transgene under wild conditions.
By considering these factors, the researchers not only could calculate the probability of genes spreading to wild populations, but also the probability that they will be passed on to future generations. Successful transgene escape, notes Haygood, depends on the survival of the gene.
With the model, the team estimated how many growing seasons it would take for artificially inserted genetic information thats been confined to fix itself in wild populations.
"This is a situation where you have chance after chance for something to happen," explains Haygood, adding, "Theres a certain chance in every generation for escape."
Because the rates of pollen flow and leakage are low, he says one would expect a long time to pass before a transgene escapes into a wild population. However, findings from the model suggest that even when the average time is as long as 100 growing seasons, the chances are that transgene escape can occur much sooner, regardless of the containment strategy.
The results show, for example, that a leakage rate of 2.5 percent - the actual value found by Hungarian scientists in the 1980s who studied the probability of chloroplast DNA transmission through the pollen of tobacco plants - could result in transgene escape within just 22 generations. Similarly, a leakage rate as low as one-tenth of 1 percent, along with plausible values for the other parameters, leaves a 60 percent chance of transgene escape within the first 10 generations.
The situation, says Haygood, is worsened when one considers that a transgenic crop is likely to be planted on more than one field, increasing the probability of escape. "Imagine that its planted not on one field, but 100. That would substantially aggravate the problem," he says.
Although the model does has some limitations, the researchers say it includes all the essential elements for predicting gene flow and can be tweaked to take into account different scenarios. "The abstract structure of the model," explains Haygood, "will be the same."
One of the key messages of the research paper, the researchers emphasize, is that scientists will need to develop containment strategies with the smallest possible leakage rate to minimize the chances of transgene escape within short periods of time.
David Andow adds, "We really need to study the failure rates of gene confinement with levels of precision perhaps on the magnitude of one out of every 10,000."
He and his colleagues from UW-Madison say that they hope this paper provides the impetus for other scientists and regulatory officials to evaluate the true effectiveness of gene containment strategies on specific crops.
Ralph Haygood | University of Wisconsin-Madison